The present disclosure is related to automatic landing systems, and more specifically to an automatic landing system with runway slope compensation.
The conventional automatic landing system (ALS) that uses radio altitude as a primary feedback for flaring to touchdown will tend to exhibit undesirable pitching activity when flown over sloping runways. This can cause excessively long or short landings and in extreme cases could cause the pilot to intervene and disconnect the ALS or abort the approach. Additionally, extreme pitch attitudes will cause risk of tailstrikes or nosewheel-first landings. Operationally, some facilities with sloping runways limit use of autoland systems in low visibility conditions due to poor performance.
The source of this undesirable pitch activity is the radio altitude signal which will include the effects of both the aircraft's movement and of changes in ground level due to the irregular runway profile. A sloping runway will cause the ALS to perceive greater or lesser vertical speeds relative to ground. If the ALS has conventional elevator based longitudinal control, the only way it can continue to follow the commanded landing profile is for the elevator to move in reaction to the altered vertical speed and pitch the aircraft. In effect, this changes the lift of the aircraft to account for the rise or fall of the runway.
A conventional ALS has no means to address this problem without pitching the aircraft using the elevator. Some mitigation for runway slope effects can be achieved through modifications of control law gains to dampen the control law's response to rapidly changing inputs. Similarly, nose down command limiting could be used to reduce pitch activity. However, changing control law gains or limiting the command to address pitch activity often results in degraded stability or performance. It does not address the underlying problem of the control law input reacting to both aircraft and to terrain movement with a single control surface. There is no way to decouple the pitch activity due to the runway slope from the rest of the flare profile.